The present invention relates to a method and device for cutting a paper web in the twin wire former of a paper machine.
Web cutting in the form of tail and/or edge cutting at the end of the forming fabric of a former is known, for instance, through U.S. Pat. Nos. 1,279,756, 2,709,398, 2,857,822 and 4,560,438.
In a twin wire former the paper web is trimmed and/or cut to a tail while being supported by one of the forming fabrics, at a point immediately before the pick-up means. A tail (also called a leader) is a strip, which is separated from the web at the wet end of a paper machine, ahead of the press section, in order to facilitate the threading of the web through the machine. For the cutting, usually water jets are used which encounter the paper web at relatively high pressure in order to form edge trimmings and a trimmed web and, when applicable, a tail, of predetermined width. One drawback is that, after slitting the paper web, the water jets spread so that water splashes all around the area. If the supporting fabric is a felt, as is the case in a crescent former, there is a considerable risk of fibers and fiber fragments being pressed into the felt and thereby giving rise to streaks in the paper web of final width when the tail cutting is complete. The pressure of the fluid jet must be limited in order to reduce the risk of the felt being damaged by the fluid jet. Repeated and extended tail cutting obviously increases said problem of fibers and fiber fragments being pressed into the felt, as well as the felt being damaged by the fluid jet even if the pressure thereof is maintained at a lower level. This in turn means that tail cutting is limited to paper webs having low basis weight such as below 30 g/m.sup.2 in uncreped state and that trimming cannot be performed at all on the felt at a point immediately before the pick-up means. If the stock contains wet strengthener, the basis weight limit is reduced still further. In order to obtain a paper web with straight edges and of a predetermined width in a crescent former, therefore, the outer forming fabric may be provided with impermeable edge portions as disclosed in U.S. Pat. No. 3,652,390. The inner, parallel edges of said edge portions, facing each other, thus determine the width of the finished paper web and said width is selected depending on the grade of soft paper to be produced. When the production of soft paper is to be changed from one grade to another, e.g. from tissue to towel paper, the width of the finished paper web must be altered in order to avoid undesirable losses during conversion of the paper web to the intended final products. To enable such alteration in the production from one grade to another, the outer forming fabric of the crescent former must be dismantled and replaced with another forming fabric with a different width between the impermeable edge portions. This exchange is troublesome and time-consuming and entails an undesirable loss of production. Furthermore, increased costs are incurred for forming fabrics as well as space to store them.